Electric double-layer capacitor

ABSTRACT

In an electrical double-layer capacitor having a pair of polarizable electrodes, a separator between the polarizable electrodes and a liquid electrolyte, the polarizable electrodes contain as a main component an activated carbon having micropores with a pore radius distribution peak as determined by the MP method within a range of 4.0×10 −10  to 8.0×10 −10  m, and the liquid electrolyte includes an electrolyte salt which is an ionic liquid. Such electrical double-layer capacitors have excellent low-temperature characteristics and a high electrostatic capacitance.

TECHNICAL FIELD

The present invention relates to electrical double-layer capacitors.More particularly, it relates to electrical double-layer capacitorshaving excellent low-temperature characteristics and a highelectrostatic capacitance.

BACKGROUND ART

Nonaqueous liquid electrolyte-type electrical double-layer capacitorscan be charged and discharged at a high current, and thus holdconsiderable promise as energy storage devices for such applications aselectric cars and auxiliary power supplies.

Prior-art nonaqueous liquid electrolyte-type electrical double-layercapacitors are constructed of a nonaqueous liquid electrolyte andpositive and negative polarizable electrodes composed largely of acarbonaceous material such as activated carbon. The composition of thenonaqueous liquid electrolyte is known to have a large influence on thewithstand voltage and electrostatic capacitance of the capacitor.

The nonaqueous liquid electrolyte is composed of an electrolyte salt anda nonaqueous organic solvent. Studies have hitherto been conducted onvarious combinations of such electrolyte salts and nonaqueous organicsolvents.

For example, quaternary ammonium salts (e.g., JP-A 61-32509, JP-A63-173312, JP-A 10-55717) and quaternary phosphonium salts (e.g., JP-A62-252927) are commonly used as the electrolyte salt because of theirsolubility and degree of dissociation in organic solvents, as well astheir broad electrochemical stability range.

Examples have also been reported in which dialkylimidazolium salts,which are ionic liquids, are used as the electrolyte salt (JP-A 6-61095,JP-A 2002-110472).

However, in electrical double-layer capacitors where solid quaternarysalts are used as the electrolyte salt, the quaternary salt readilydeposits out of solution at low temperatures, and particularly at verylow temperatures of −20° C. or less. Even in the absence of suchdeposition, the electrical conductivity falls off dramatically at lowtemperatures.

When dialkylimidazolium salts, as ionic liquids, are used to resolvethis problem, mixed systems of these salts with inorganic salts are verysensitive to such factors as humidity in the air, and are thus difficultto handle. Moreover, imidazolium salts themselves have drawbacks; namelythey have melting points which are not as low as might be desired, andthey also have a relatively narrow potential window.

The polarizable electrodes are generally composed of activated carbon.This activated carbon is made by carbonizing any of various suitablestarting materials, examples of which include natural substances such ascoconut shells and sawdust, synthetic resins such as phenolic resins andpolyimide resins, and also coal- and petroleum-based pitch, mesophasecarbon, carbon fibers and discarded tires. The carbonized material isthen activated, such as by gas activation with steam or carbon dioxide,or by chemical activation using zinc chloride, potassium hydroxide orphosphoric acid. The larger the specific surface area of the activatedcarbon, the greater the electrostatic capacitance tends to be, althoughthis relationship has yet to be fully investigated.

It is therefore one object of the invention to provide electricaldouble-layer capacitors endowed with excellent low-temperaturecharacteristics and a high electrostatic capacitance.

DISCLOSURE OF THE INVENTION

In order to achieve the above object, the inventors have conductedextensive investigations on the relationship at low temperatures betweenthe electrolyte salt and the pore distribution in the activated carbon.As a result, they have found that when quaternary ammonium salts andquaternary phosphonium salts bearing at least one alkoxyalkyl group as asubstituent are used as the electrolyte salt, and when the activatedcarbon making up the polarizable electrodes is one having microporeswith a pore radius distribution peak as determined by what is known asthe “MP method” within a specific range, electrical double-layercapacitors can be obtained which have, at low temperatures, excellentcharge-discharge characteristics and a low internal impedance.

Accordingly, the invention provides the following:

-   (1) An electrical double-layer capacitor having a pair of    polarizable electrodes, a separator between the polarizable    electrodes and a liquid electrolyte, which electrical double-layer    capacitor is characterized in that the polarizable electrodes    contain as a main component an activated carbon having micropores    with a pore radius distribution peak as determined by the method    within a range of 4.0×10⁻¹⁰ to 8.0×10⁻¹⁰ m, and the liquid    electrolyte includes an electrolyte salt which is an ionic liquid.-   (2) The electrical double-layer capacitor of (1) above which is    characterized in that the ionic liquid is a quaternary ammonium salt    or a quaternary phosphonium salt.-   (3) The electrical double-layer capacitor of (2) above which is    characterized in that the ionic liquid has general formula (1) below    wherein R¹ to R⁴ are each independently an alkyl group of 1 to 5    carbons or an alkoxyalkyl group of the formula R′—O—(CH₂)_(n)— (R′    being methyl or ethyl, and the letter n being an integer from 1 to    4), and any two from among R¹, R², R³ and R⁴ may together form a    ring, with the proviso that at least one of R¹ to R⁴ is the    alkoxyalkyl group of the above formula; X is a nitrogen atom or a    phosphorus atom; and Y is a monovalent anion.-   (4) The electrical double-layer capacitor of (3) above which is    characterized in that the ionic liquid has general formula (2) below    wherein Me stands for methyl and Et stands for ethyl.-   (5) The electrical double-layer capacitor of any one of (1) to (4)    above which is characterized in that the pore radius distribution    peak is in a range of 4.5×10⁻¹⁰ to 7.0×10⁻¹⁰.-   (6) The electrical double-layer capacitor of any one of (1) to (5)    above which is characterized in that the ionic liquid has a    concentration in the liquid electrolyte of from 0.5 to 2.0 mol/L.-   (7) The electrical double-layer capacitor of any one of (1) to (6)    above which is characterized in that the activated carbon is an    activated form of a synthetic resin.-   (8) The electrical double-layer capacitor of (7) above which is    characterized in that the activated carbon is a steam-activated form    of a synthetic resin.-   (9) The electrical double-layer capacitor of (7) or (8) above which    is characterized in that the synthetic resin is a phenolic resin    and/or a polycarbodiimide resin.

BRIEF DESCRIPTION OF THE DIAGRAMS

FIG. 1 is a chart showing the NMR spectrum of compound (2).

FIG. 2 is a chart showing the NMR spectrum of compound (11).

BEST MODE FOR CARRYING OUT THE INVENTION

The invention is described more fully below.

As described above, the electrical double-layer capacitor according tothis invention has a pair of polarizable electrodes, a separator betweenthe polarizable electrodes and a liquid electrolyte, and ischaracterized in that the polarizable electrodes contain as a maincomponent an activated carbon having micropores with a pore radiusdistribution peak as determined by the MP method within a range of4.0×10⁻¹⁰ to 8.0×10⁻¹⁰ m, and the liquid electrolyte includes anelectrolyte salt which is an ionic liquid.

The ionic liquid, although not subject to any particular limitation, ispreferably a quaternary ammonium salt or a quaternary phosphonium salt,and most preferably an ionic liquid of general formula (1) below.

In formula (1), R¹ to R⁴ are each independently an alkyl group of 1 to 5carbons or an alkoxyalkyl group of the formula R′—O—(CH₂)_(n)— (R′ beingmethyl or ethyl, and the letter n being an integer from 1 to 4), and anytwo from among R¹, R², R³ and R⁴ may together form a ring, with theproviso that at least one of R¹ to R⁴ is the alkoxyalkyl group of theabove formula. X is a nitrogen atom or a phosphorus atom, and Y is amonovalent anion.

Examples of alkyls having 1 to 5 carbons include methyl, ethyl, propyl,2-propyl, butyl and pentyl. However, given that a smaller ionic radiusaffords better ionic mobility within the liquid electrolyte, it ispreferable for at least one of groups R¹ to R⁴ to be methyl, ethyl orpropyl, and especially methyl or ethyl. The ethyl or propyl group mayform a ring with another alkyl group.

Examples of alkoxyalkyl groups of the formula R′—O—(CH₂)_(n)— includemethoxymethyl, ethoxymethyl, methoxyethyl, ethoxyethyl, methoxypropyl,ethoxypropyl, methoxybutyl and ethoxybutyl. The letter n is an integerfrom 1 to 4. However, to increase the stability of the ionic liquid, theletter n is preferably 1 or 2, and most preferably 2.

Exemplary compounds in which any two groups from among R¹ to R⁴ form aring include, when X is a nitrogen atom, quaternary ammonium saltscontaining an aziridine, azetidine, pyrrolidine or piperidine ring; and,when X is a phosphorus atom, quaternary phosphonium salts containing apentamethylenephosphine (phosphorinane) ring.

Specific examples of quaternary ammonium salts and quaternaryphosphonium salts highly suitable for use in the invention includecompounds (2) to (11) below (wherein Me stands for methyl and Et standsfor ethyl). The quaternary salt of formula (2) below, which bears assubstituents a methyl group, two ethyl groups and a methoxyethyl groupand which includes as the anion BF₄ ⁻, is especially preferred. The useof this ionic liquid as the electrolyte salt enables electricaldouble-layer capacitors having excellent charge-dischargecharacteristics at even lower temperatures to be obtained.

Illustrative, non-limiting examples of the monovalent anion Y includeBF₄ ⁻, PF₆ ⁻, AsF₆ ⁻, SbF₆ ⁻, AlC₄ ⁻, HSO₄ ⁻, ClO₄ ⁻, CH₃SO₃ ⁻, CF₃SO³⁻, CF₃CO₂ ⁻, (CF₃SO₂)₂N⁻, Cr—, Br— and I—. To provide such properties asa good degree of dissociation and good stability in nonaqueous organicsolvents, the use of BF₄ ⁻, PF₆ ⁻, (CF₃SO₂)₂N⁻, CF₃SO₃ ⁻ or CF₃CO₂ ⁻ ispreferred.

Of these anions, the use of (CF₃SO₂)₂N⁻ is highly preferable for furtherreducing the viscosity of the ionic liquid and increasing itshandleability. BF₄ ⁻ is also highly preferable because the resultingionic liquid has a high versatility and it is less readily affected bywater than ionic liquids containing PF₆ ⁻ as the anion and thus easierto handle.

A common method for synthesizing such quaternary ammonium salts isdescribed. First, a tertiary amine is mixed with a compound such as analkyl halide or a dialkyl sulfate and reacted under heating, ifnecessary, to give a quaternary ammonium halide. In cases where acompound having a low reactivity (e.g., an alkoxyethyl halide or analkoxymethyl halide) is used, reaction under applied pressure, such asin an autoclave, is preferred.

The resulting quaternary ammonium halide is dissolved in an aqueoussolvent such as water, then reacted with a reagent that generates therequired anionic species, such as tetrafluoroboric acid ortetrafluorophosphoric acid, so as to effect an anion exchange reaction,yielding the quaternary ammonium salt. Alternatively, if the quaternaryammonium halide is soluble in an organic solvent, the halide may bereacted with a silver salt of the required anionic species so as toeffect an anion exchange reaction and thereby yield the quaternaryammonium salt.

In one illustrative method for synthesizing quaternary ammoniumtetrafluoroborates, a quaternary ammonium halide is dissolved in water,silver oxide is added and a salt exchange reaction is carried out toform the corresponding quaternary ammonium hydroxide. The product isthen reacted with tetrafluoroboric acid, yielding the target compound.This method is effective for synthesizing high-purity quaternaryammonium tetrafluoroborates because the silver halide that arises as aresult of salt exchange during formation of the quaternary ammoniumhydroxide can easily be removed.

Quaternary phosphonium salts can generally be synthesized in much thesame way as quaternary ammonium salts. Typically, a tertiary phosphineis mixed with a suitable compound such as an alkyl halide or a dialkylsulfate. If necessary, the reaction is carried out under the applicationof heat.

As in the case of quaternary ammonium salts, quaternary phosphoniumsalts containing any of various suitable anions may be prepared bydissolving a quaternary phosphonium halide (a chloride, bromide oriodide) in an aqueous solvent and reacting the dissolved halide with areagent that generates the required anionic species so as to effect ananion exchange reaction.

The above ionic liquid has a melting point not higher than 50° C.,preferably not higher than 30° C., and most preferably not higher than20° C. If the melting point is higher than 50° C., the ionic liquid willdeposit out within the electrolyte at low temperatures, increasing thelikelihood of a decline in the ionic conductivity. The lower the meltingpoint, the more desirable. The melting point has no particular lowerlimit.

Because the above-described ionic liquid has a lower melting point thanimidazolium ion-containing ionic liquids already familiar to the art, byusing an electrolyte containing the above ionic liquid, there can beobtained electrical double-layer capacitors having even betterlow-temperature characteristics.

Also, because the above ionic liquid has a broader potential window thanionic liquids containing imidazolium ions, it does not readily undergoreductive decomposition during charging and discharging. As a result, ahighly stable electrical double-layer capacitor can be obtained.

The liquid electrolyte of the inventive electrical double-layercapacitor includes an ionic liquid and a nonaqueous organic solvent. Anynonaqueous organic solvent which is capable of dissolving the ionicliquid and is stable within the working voltage range of the electricaldouble-layer capacitor may be used without particular limitation.However, it is preferable for the nonaqueous organic solvent to be onehaving a high dielectric constant, a broad electrochemical stabilityrange, a broad service temperature range and excellent safety.

Illustrative examples of suitable solvents include nitrites such asacetonitrile and propionitrile; acyclic ethers such as dibutyl ether,1,2-dimethoxyethane, 1,2-ethoxymethoxyethane, methyl diglyme, methyltriglyme, methyl tetraglyme, ethyl glyme, ethyl diglyme, butyl diglyme,and glycol ethers (e.g., ethyl cellosolve, ethyl carbitol, butylcellosolve, butyl carbitol); heterocyclic ethers such astetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane and4,4-dimethyl-1,3-dioxane; butyrolactones such as γ-butyrolactone,γ-valerolactone, δ-valerolactone, 3-methyl-1,3-oxazolidin-2-one and3-ethyl-1,3-oxazolidin-2-one; and other solvents commonly used inelectrochemical devices, such as amide solvents (e.g.,N-methylformamide, N,N-dimethylformamide, N-methylacetamide,N-methylpyrrolidinone), carbonate solvents (e.g., diethyl carbonate,dimethyl carbonate, ethyl methyl carbonate, propylene carbonate,ethylene carbonate, styrene carbonate), and imidazolidinone solvents(e.g., 1,3-dimethyl-2-imidazolidinone). Any one or mixtures of two ormore of these solvents may be used.

Of these solvents, propylene carbonate is especially preferred becauseit has a good ability to dissolve the electrolyte salt even at very lowtemperatures of −20° C. or less, an excellent electrical performance,and a relatively high flash point during use at elevated temperatures.

No particular limitation is imposed on the concentration of the ionicliquid within the liquid electrolyte, although the concentration ispreferably 0.5 to 2.0 mol/L, more preferably 0.75 to 1.75 mol/L, andeven more preferably 0.9 to 1.5 mol/L.

At an ionic liquid concentration of less than 0.5 mol/L, energy loss mayoccur due to increased internal resistance. On the other hand, at aconcentration of more than 2.0 mol/L, undesirable effects may arise,such an increase in viscosity and a decrease in electrical conductivity.

It is also possible to use the ionic liquid alone as the liquidelectrolyte without using a solvent.

The polarizable electrodes used in the electrical double-layer capacitorof the invention contain as a main component an activated carbon havingmicropores with a pore radius distribution peak as determined by the MPmethod within a range of 4.0×10⁻¹⁰ to 8.0×10⁻¹⁰ m (4.0 to 8.0 Å),preferably 4.5×10⁻¹⁰ to 7.0×10⁻¹⁰ m (4.5 to 7.0 Å), more preferably4.5×10⁻¹⁰ to 6.5×10⁻¹⁰ m (4.5 to 6.5 Å), and even more preferably4.5×10⁻¹⁰ to 5.0×10⁻¹⁰ m (4.5 to 5.0 Å).

“MP method,” as used herein, refers to a method commonly used inmicropore analysis. The results of BET measurements are t-plotted andthe curvature in areas near bends in the plot is analyzed, based uponwhich the pore radius distribution and the pore radius distribution peakare computed. The pore radius distribution and peak here are valuesdetermined from the results of BET measurements based on nitrogenadsorption.

If the resulting peak in the pore radius distribution for micropores inthe activated carbon is lower than 4.0×10⁻¹⁰, the charge-dischargecharacteristics at low temperatures may worsen. On the other hand, at apore radius distribution peak higher than 8.0×10⁻¹⁰, maintaining a largespecific surface area in the activated carbon is difficult. Accordingly,the electrostatic capacitance may decrease on account of the smallerspecific surface area.

The activated carbon may be prepared from any suitable startingmaterial, provided the pore radius distribution peak can be set withinthe above-indicated range. Starting materials that may be used includecoconut shells, coffee beans, sawdust, petroleum pitch, petroleum cokeand coal, although it is preferable for the activated carbon to beprepared by carbonizing a synthetic resin as the starting material, thenactivating the carbonized resin.

Any of various synthetic resins known to be suitable as startingmaterials for producing activated carbon may be used. Illustrativeexamples include synthetic polymers, phenolic resins, furan resins,polyvinyl chloride resins, polyvinylidene chloride resins, polyimideresins, polyamide resins, liquid crystal polymers, plastic waste anddiscarded tires. From the standpoint of cost, versatility, and ease ofactivation, phenolic resins and/or polycarbodiimide resins arepreferred.

The method of activation is not subject to any particular limitation.Examples of such techniques that may be used include chemical activationusing a suitable chemical such as potassium hydroxide, zinc chloride orphosphoric acid, and gas activation using a suitable gas such as carbondioxide, oxygen or steam. Steam activation is preferred because the poreradius of the activated carbon can easily be controlled within theabove-indicated range.

Activated carbon exists in a variety of forms, including crushedmaterial, granulated material, pellets, fibers, felt, woven fabric andsheets, any of which may be used in the invention.

The above-described polarizable electrodes are largely composed ofactivated carbon having the above-indicated pore radius distribution,and may be of a type fabricated by blending a binder polymer into thisactivated carbon to form a polarizable electrode composition, thencoating the composition onto a current collector.

Here, the binder polymer may be any known binder polymer capable ofbeing used in applications to which this invention relates. Illustrativeexamples include polytetrafluoroethylene, polyvinylidene fluoride,carboxymethyl cellulose, fluoroolefin copolymer-crosslinked polymers,polyvinyl alcohols, polyacrylic acids, polyimides, petroleum pitch, coalpitch, and phenolic resins.

These binder polymers are preferably added in any amount of 0.5 to 20parts by weight, and especially 1 to 10 parts by weight, per 100 partsby weight of the activated carbon.

The method of preparing the polarizable electrode composition is notsubject to any particular limitation. For example, the composition maybe prepared in the form of a solution from the above-described activatedcarbon and a binder polymer, or it may be prepared by adding a solvent,if necessary, to this solution.

The polarizable electrode composition thus obtained is coated onto acurrent collector to form a polarizable electrode. Any suitable knowncoating method may be used at this time, such as doctor blade coating orair knife coating.

Any current collector commonly used in electrical double-layercapacitors may be selected for use as the current collector in thepositive and negative electrodes. The positive electrode currentcollector is preferably aluminum foil or aluminum oxide, and thenegative electrode current collector is preferably copper foil, nickelfoil or a metal foil covered on the surface with a copper plating filmor a nickel plating film.

The foils making up the respective current collectors may be in any ofvarious forms, including thin foils, flat sheets, and perforated,stampable sheets. The foil has a thickness of generally about 1 to 200μm. However, taking into account, for example, the density of theactivated carbon over the entire electrode and the strength of theelectrode, a thickness of 8 to 100 μm is preferred, and a thickness of 8to 30 is especially preferred.

Alternatively, the polarizable electrodes can be fabricated by meltingand blending the polarizable electrode composition, then extruding theblend as a film.

A conductive material may be added to the above-described carbonaceousmaterial. The conductive material may be any suitable material capableof conferring electrical conductivity to the carbonaceous material.Illustrative, non-limiting, examples include carbon black, Ketjenblack,acetylene black, carbon whiskers, carbon fibers, natural graphite,artificial graphite, titanium oxide, ruthenium oxide, and metallicfibers such as aluminum or nickel fibers. Any one or combinations of twoor more thereof may be used. Of these, Ketjenblack and acetylene black,both of which are types of carbon black, are preferred.

The average particle size of the conductive material is not subject toany particular limitation, although a size of 10 nm to 10 μm, preferably10 to 100 nm, and more preferably 20 to 40 nm, is desirable. Inparticular, it is advantageous for the conductive material to have anaverage particle size which is from 1/5000 to 1/2, and preferably from1/1000 to 1/10, as large as the average particle size of the activatedcarbon.

The amount of conductive material included is not subject to anyparticular limitation, although addition of the conductive material inan amount of 0.1 to 20 parts by weight, and preferably 0.5 to 10 partsby weight, per 100 parts by weight of the activated carbon is desirablein light of such considerations as the electrostatic capacitance and theconductivity-imparting effects.

The separator may be one that is commonly used in electricaldouble-layer capacitors. Illustrative examples include polyolefinnonwoven fabric, polytetrafluoroethylene porous film, kraft paper, sheetlaid from a blend of rayon fibers and sisal hemp fibers, manila hempsheet, glass fiber sheet, cellulose-based electrolytic paper, paper madefrom rayon fibers, paper made from a blend of cellulose and glassfibers, and combinations thereof in the form of multilayer sheets.

The electrical double-layer capacitor of the invention can be assembledby stacking, fan-folding or winding an electrical double-layer capacitorassembly composed of a pair of polarizable electrodes produced asdescribed above and a separator therebetween. The cell assembly is thenplaced within a capacitor housing such as a can or a laminate pack.Next, the assembly is filled with the liquid electrolyte, followingwhich the housing is mechanically sealed if it is a can or heat-sealedif it is a laminate pack.

The electrical double-layer capacitors of the invention are highlysuitable for use as a memory backup power supply for cell phones,notebook computers and wireless terminals, as a power supply for cellphones and portable acoustic devices, as an uninterruptible power supplyfor personal computers and other equipment, and as various types oflow-current electrical storage devices such as load leveling powersupplies used in combination with solar power generation and wind powergeneration. Moreover, electrical double-layer capacitors capable ofbeing charged and discharged at a high current are well suited for useas high-current electrical storage devices in applications that requirea large current such as electric cars and electrical power tools.

As described above, because the electrical double-layer capacitor of theinvention uses polarizable electrodes that are largely composed ofactivated carbon in which the micropores have a specific pore radiusdistribution peak, and because it uses a liquid electrolyte thatcontains an ionic liquid, the capacitor has excellent charge-dischargecharacteristics at low temperatures and the internal impedance at lowtemperatures can be minimized.

Moreover, because ionic liquids composed of a quaternary ammonium saltor quaternary phosphonium salt have a broader potential window thanimidazolium or pyridinium-type ionic liquids, the use of such ionicliquids as the electrolyte enables electrical double-layer capacitors ofa high energy density to be obtained.

EXAMPLE

The following synthesis examples, examples of the invention andcomparative examples are provided to illustrate the invention and do notin any way limit the invention.

Synthesis Example 1

Synthesis of Compound (2)

A solution prepared by mixing together 100 ml of diethylamine (KantoChemical Co., Inc.) and 85 ml of 2-methoxyethyl chloride (Kanto ChemicalCo. Inc.) was placed in an autoclave and reacted at 100° C. for 24hours. The internal pressure during the reaction was 0.127 MPa (1.3kgf/cm²). This yielded a mixture of deposited crystals and reactionsolution to which was added, following the 24 hours of reaction, 200 mlof an aqueous solution containing 56 g of dissolved calcium hydroxide(Katayama Chemical Industries Co., Ltd.). The two organic phases thatformed as a result were separated with a separatory funnel and subjectedtwice to extraction with 100 ml of methylene chloride (Wako PureChemical Industries, Ltd.). The separated organic phases were thencombined and washed with a saturated saline solution, following whichpotassium carbonate (Wako Pure Chemical Industries, Ltd.) was added toremove water, and vacuum filtration was carried out. The solvent in theresulting organic phase was distilled off using a rotary evaporator,after which the residue was subjected to normal-pressure distillation,yielding 18.9 g of a fraction having a boiling point close to 135° C.This compound was confirmed from a ¹H-nuclear magnetic resonance(abbreviated hereinafter as “NMR”) spectrum to be2-methoxyethyldiethylamine.

Next, 8.24 g of the 2-methoxyethyldiethylamine was dissolved in 10 ml oftetrahydrofuran (Wako Pure Chemical Industries, Ltd.), then 4.0 ml ofmethyl iodide (Wako Pure Chemical Industries, Ltd.) was added under icecooling. After 30 minutes, the mixture was removed from the ice bath andstirred overnight at room temperature. The solvent in the resultingreaction solution was subsequently driven off by vacuum distillation,and the resulting solids were recrystallized from an ethanol (Wako PureChemical Industries, Ltd.)—tetrahydrofuran system, yielding 16 g of2-methoxyethyldiethylmethylammonium iodide.

Next, 15.0 g of the 2-methoxyethyldiethylmethyl-ammonium iodide wasdissolved in 100 ml of distilled water, following which 6.37 g of silveroxide (Kanto Chemical Co. Inc.) was added and stirring carried out for 3hours. The reaction mixture was then vacuum filtered to remove theprecipitate, following which 42% tetrafluoroboric acid (Kanto ChemicalCo. Inc.) was gradually added under stirring until the reaction solutionreached a pH of about 5 to 6. This reaction solution was subsequentlyfreeze-dried, in addition to which water was thoroughly driven off usinga vacuum pump, yielding 12.39 g of a compound (2) that was liquid atroom temperature (25° C.).

FIG. 1 shows the NMR spectrum (solvent: deuterated chloroform) ofcompound (2).

Synthesis Example 2

Synthesis of Compound (11)

First, 100 ml of a 2.0 M dimethylamine-tetrahydrofuran solution (AldrichChemical Co., Ltd.) and 9.1 ml of 2-methoxyethyl chloride (KantoChemical Co. Inc.) were mixed, and the mixture was reacted in anautoclave at 100° C. for 12 hours. The internal pressure during thereaction was 0.36 MPa (3.7 kgf/cm²). The crystals that had formed in thereaction solution after 12 hours of reaction were filtered off, and thefiltrate was subjected to distillation so as to remove most of thetetrahydrofuran, thereby giving a clear liquid that was adimethyl-2-methoxyethyl mixture.

Next, 8.0 ml of methyl iodide (Wako Pure Chemical Industries, Ltd.) wasadded to this liquid under ice cooling, following which the ice bath wasremoved and the mixture was stirred overnight. The resulting reactionmixture was vacuum distilled, giving 3.04 g of the salt2-methoxyethylethyldimethylammonium iodide as an oil.

Next, 2.28 g of silver tetrafluoroborate was weighed out, 30 ml of a 1:1(by volume) chloroform-acetonitrile mixed solvent was added and themixture was stirred. To the resulting suspension was added a solution of3.04 g of the 2-methoxyethyldimethylammonium iodide prepared above in 30ml of 1:1 chloroform-acetonitrile, and the resulting mixture was stirredfor 80 minutes. The crystals that formed were removed by vacuumfiltration, following which the solvent within the filtrate was drivenoff with an evaporator and a vacuum pump.

Next, 2.85 g of the residue was purified by silica gel columnchromatography using Wakogel (C-200, produced by Wako Pure ChemicalIndustries, Ltd.) and a 1:1 (by volume) mixture of chloroform andmethanol as the eluting solvent, yielding 1.57 g of compound (11) whichwas liquid at room temperature (25° C.).

FIG. 2 shows the NMR spectrum (solvent: deuterated dimethyl sulfoxide)of compound (11).

Example 1

A filling material was prepared by mixing an activated carbon (indicatedbelow as “Activated Carbon 1”) obtained by steam activating a carbonizedphenolic resin for 2 hours and having the specific surface area and thepore distribution peak value shown in Table 1, a conductive material(HS-100, made by Denki Kagaku Kogyo Kabushiki Kaisha) and a binder(PVdF900, made by Kureha Chemical Industries Co., Ltd.) in a weightratio of 90:5:5, respectively. This filling material was then mixed withN-methyl-2-pyrrolidone (abbreviated hereinafter as “NMP”; made byKatayama Chemical Industries Co., Ltd.) in a weight ratio (fillingmaterial/NMP) of 100:212.5 to form a slurry.

The slurry was applied onto 30 μm aluminum foil to an electrodethickness of 100 μm, vacuum dried at 140° C. for 3 days, then subjectedto 30 MPa of stress using a roll press.

The slurry-coated aluminum foil was vacuum dried once again at 170° C.for 3 days, following which 12 mm diameter electrodes were punched on apunching machine. The punched disks were then vacuum dried at 120° C.for 2 hours, thereby giving test electrodes.

Next, using a two-electrode coin cell housing (Hokuto DenkoCorporation), a coin cell was assembled from the above-describedelectrodes together with an intervening cellulose separator (FT40-35,made by Nippon Kodoshi Corporation) and a 1.0 M propylene carbonate(abbreviated hereinafter as “PC”; made by Kishida Chemical Co., Ltd.)solution of Compound (2) as the liquid electrode so as to form anelectrical double-layer capacitor sample.

Example 2

Aside from using as the activated carbon in the electrodes an activatedcarbon (indicated below as “Activated Carbon 2”) prepared by steamactivating carbonized phenolic resin for 3 hours and using Compound (11)as the electrolyte salt in the liquid electrolyte, an electricaldouble-layer capacitor sample was fabricated in the same way as inExample 1.

Example 3

Aside from using as the activated carbon within the electrodes anactivated carbon (indicated below as “Activated Carbon 3”) prepared bysubjecting alkali-activated carbon (MSP-20, produced by Kansai Coke andChemicals Co., Ltd.) to 1 hour of steam activation as well so as tobroaden the pore size distribution, an electrical double-layer capacitorsample was fabricated in the same way as in Example 1.

Example 4

Aside from using as the activated carbon within the electrodes anactivated carbon (indicated below as “Activated Carbon 3”) prepared bysubjecting alkali-activated carbon (MSP-20, made by Kansai Coke andChemicals Co., Ltd.) to 1 hour of steam activation as well so as tobroaden the pore size distribution and using Compound (11) as theelectrolyte salt within the liquid electrolyte, an electricaldouble-layer capacitor sample was fabricated in the same way as inExample 1.

Comparative Example 1

Aside from using as the activated carbon in the electrodes analkali-activated carbon (MSP-20, made by Kansai Coke and Chemicals Co.,Ltd.) that had not been further activated (indicated below as “ActivatedCarbon 4”), an electrical double-layer capacitor sample was fabricatedin the same way as in Example 1.

Comparative Example 2

Aside from using as the liquid electrolyte a 1.0 M solution(LIPASTE-P/EAFIN, produced by Toyama Chemical Co., Ltd.) oftetraethylammonium tetrafluoroborate (TEA) in propylene carbonate, anelectrical double-layer capacitor sample was fabricated in the same wayas in Example 1.

Comparative Example 3

Aside from using as the activated carbon in the electrodes an activatedcarbon prepared by subjecting carbonized phenolic resin to 6 hours ofsteam activation (indicated below as “Activated Carbon 5”), anelectrical double-layer capacitor sample was fabricated in the same wayas in Example 1. TABLE 1 BET specific surface area Pore radius peakvalue Activated carbon (m²/g) (Å) Activated Carbon 1 1642 4.64 ActivatedCarbon 2 2120 6.30 Activated Carbon 3 2201 4.95 Activated Carbon 4 22803.74 Activated Carbon 5 1620 8.31

The characteristics of the electrical double-layer capacitors obtainedin each of the above examples of the invention and comparative exampleswere evaluated as described below when charged and discharged in acharge/discharge system (1005SM8, manufactured by Hokuto DenkoCorporation).

The method of evaluation was an initial capacitance verification test,carried out in a room temperature environment, in which the capacitorwas charged at a current density of 0.88 mA/cm², a voltage setting of2.50 V and a constant voltage time of 15 minutes (cut-off conditions),and was discharged at a current density of 0.88 mA/cm² and anend-of-discharge voltage of 0.0 V.

A low-temperature performance test was carried out in which the testcell was placed within a constant temperature chamber (EC-25MTP,manufactured by Hitachi, Ltd.) and held at −40° C. for about 6 hours,following which charge and discharge were carried out under the samecurrent and voltage conditions as in the initial capacitanceverification test. The results obtained from these evaluation tests areshown below in Table 2. TABLE 2 Retention of Activated ElectrolyteCapacitance capacitance carbon salt (F/g) (%) Example 1 ActivatedCompound 2 23.4 90 Carbon 1 Example 2 Activated Compound 11 25.9 95Carbon 2 Example 3 Activated Compound 2 33.6 93 Carbon 3 Example 4Activated Compound 11 33.5 93 Carbon 3 Comparative Activated Compound 235.0 48 Example 1 Carbon 4 Comparative Activated TEA 24.2 65 Example 2Carbon 1 Comparative Activated Compound 2 21.6 99 Example 3 Carbon 5

As is apparent from Table 2, the electrical double-layer capacitorsobtained in Examples 1 to 4 of the invention using activated carbonshaving a specific pore radius distribution as a main component of thepolarizable electrodes and using a quaternary ammonium salt (ionicliquid) as the electrolyte salt all exhibited much better retention ofthe capacitance at −40° C. than did the electrical double-layercapacitors obtained in the comparative examples. Moreover, theelectrical double-layer capacitors obtained in Examples 1 to 4 also hadsufficiently high electrostatic capacitances.

Because the electrical double-layer capacitors according to thisinvention use polarizable electrodes which are largely composed of anactivated carbon having micropores with a pore radius distribution peakas determined by the MP method within a range of 4.0×10⁻¹⁰ to 8.0×10⁻¹⁰m and use a liquid electrolyte that includes an electrolyte salt whichis an ionic liquid, they have excellent charge-discharge characteristicsat low temperatures and their internal impedance at low temperatures canbe minimized.

1. An electrical double-layer capacitor comprising a pair of polarizableelectrodes, a separator between the polarizable electrodes and a liquidelectrolyte, which electrical double-layer capacitor is characterized inthat the polarizable electrodes contain as a main component an activatedcarbon having micropores with a pore radius distribution peak asdetermined by the MP method within a range of 4.0×10⁻¹⁰ to 8.0×10⁻¹⁰ m,and the liquid electrolyte includes an electrolyte salt which is anionic liquid.
 2. The electrical double-layer capacitor of claim 1 whichis characterized in that the ionic liquid is a quaternary ammonium saltor a quaternary phosphonium salt.
 3. The electrical double-layercapacitor of claim 2 which is characterized in that the ionic liquid hasgeneral formula (1) below

wherein R¹ to R⁴ are each independently an alkyl group of 1 to 5 carbonsor an alkoxyalkyl group of the formula R′—O—(CH₂)_(n)— (R′ being methylor ethyl, and the letter n being an integer from 1 to 4), and any twofrom among R¹, R², R³ and R⁴ may together form a ring, with the provisothat at least one of R¹ to R⁴ is the alkoxyalkyl group of the aboveformula; X is a nitrogen atom or a phosphorus atom; and Y is amonovalent anion.
 4. The electrical double-layer capacitor of claim 3which is characterized in that the ionic liquid has general formula (2)below

wherein Me stands for methyl and Et stands for ethyl.
 5. The electricaldouble-layer capacitor of any one of claims 1 to 4 which ischaracterized in that the pore radius distribution peak is in a range of4.5×10⁻¹⁰ to 7.0×10⁻¹⁰.
 6. The electrical double-layer capacitor ofclaim 1 which is characterized in that the ionic liquid has aconcentration in the liquid electrolyte of from 0.5 to 2.0 mol/L.
 7. Theelectrical double-layer capacitor of claim 1 which is characterized inthat an activated carbon is the activated form of a synthetic resin. 8.The electrical double-layer capacitor of claim 7 which is characterizedin that the activated carbon is a steam-activated form of a syntheticresin.
 9. The electrical double-layer capacitor of claim 7 which ischaracterized in that the synthetic resin is a phenolic resin and/or apolycarbodiimide resin.